This invention is directed to optical devices and related methods. More particularly, the invention provides a method and device for emitting electromagnetic radiation using non-polar gallium containing substrates, such as GaN, MN, InN, InGaN, AlGaN, and AlInGaN, and others. By way of example, the invention can be applied to optical devices, lasers, light emitting diodes, solar cells, photoelectrochemical water splitting and hydrogen generation, photodetectors, integrated circuits, and transistors, among other devices.
In the late 1800's, Thomas Edison invented the light bulb. The conventional light bulb, commonly called the “Edison bulb,” has been used for over one hundred years for a variety of applications, including lighting and displays. The conventional light bulb uses a tungsten filament enclosed in a glass bulb sealed in a base, which is screwed into a socket. The socket is coupled to an AC power or DC power source. Such light bulbs can be found commonly in houses, buildings, and outdoor lighting displays, as well as elsewhere.
Unfortunately, there are several disadvantages to the conventional Edison light bulb. First, the conventional light bulb dissipates considerable energy as thermal energy. More than 90% of the energy used for the conventional light bulb is lost as thermal energy. Secondly, reliability is a concern. The conventional light bulb routinely fails from thermal expansion and contraction of the filament element. Furthermore, light bulbs emit light over a broad spectrum, much of which does not result in bright illumination due to the spectral sensitivity of the human eye. Another disadvantage is that light bulbs emit light in all directions. Therefore they are not ideal for applications requiring directionality or focus, such as projection displays, optical data storage, or specialized directed lighting.
In 1960, the laser was first demonstrated by Theodore H. Maiman at Hughes Research Laboratories in Malibu. This laser utilized a solid-state flashlamp-pumped synthetic ruby crystal to produce red laser light at 694 nm. By 1964, blue and green laser output was demonstrated by William Bridges at Hughes Aircraft utilizing a gas Argon ion laser. The Ar-ion laser utilized a noble gas as the active medium and produced laser light output in the UV, blue, and green wavelengths including 351 nm, 454.6 nm, 457.9 nm, 465.8 nm, 476.5 nm, 488.0 nm, 496.5 nm, 501.7 nm, 514.5 nm, and 528.7 nm. The Ar-ion laser had the benefit of producing highly directional and focusable light with a narrow spectral output, but the wall plug efficiency was <0.1%. In addition, the size, weight, and cost of the lasers were undesirable.
As laser technology evolved, more efficient lamp pumped solid state laser designs were developed for the red and infrared wavelengths, but these technologies remained a challenge for blue and green and blue lasers. As a result, lamp pumped solid state lasers were developed in the infrared, with the output wavelength was converted to the visible using specialty crystals with nonlinear optical properties. A green lamp pumped solid state laser had 3 stages: electricity powers lamp, lamp excites gain crystal which lases at 1064 nm, 1064 nm goes into frequency conversion crystal which converts to visible 532 nm. The resulting green and blue lasers were called “lamped pumped solid state lasers with second harmonic generation” (LPSS with SHG) had wall plug efficiency of ˜1%, and were more efficient than Ar-ion gas lasers, but were still too inefficient, large, expensive, fragile for broad deployment outside of specialty scientific and medical applications. Additionally, the gain crystal used in the solid state lasers typically had energy storage properties which made the lasers difficult to modulate at high speeds which limited its broader deployment.
To improve the efficiency of these visible lasers, high power diode (or semiconductor) lasers are now widely utilized. These “diode pumped solid state lasers with SHG” (DPSS with SHG) had 3 stages: electricity powers 808 nm diode laser, 808 nm excites gain crystal which lases at 1064 nm, 1064 nm goes into frequency conversion crystal which converts to visible 532 nm light. The DPSS laser technology extended the life and improved the wall plug efficiency of the LPSS lasers to 5-10%. Further commercialization resulted in more high end specialty industrial, medical, and scientific applications. The change to diode pumping, however, increases the system cost and requires precise temperature control. The resulting laser is large, consumes substantial power, yet does not address the energy storage properties which make the lasers difficult to modulate at high speeds.
As high power laser diodes evolved and new specialty SHG crystals were developed, it became possible to directly convert the output of the infrared diode laser to provide blue and green laser light output. These “directly doubled diode lasers” or SHG diode lasers had 2 stages: electricity powers 1064 nm semiconductor laser, 1064 nm goes into frequency conversion crystal which converts to visible 532 nm green light. These lasers designs improve the efficiency, cost and size compared to DPSS—SHG lasers, but the specialty diodes and crystals required make this challenging today. Additionally, while the diode-SHG lasers have the benefit of being directly modulated, they suffer from sensitivity to temperature, limiting their application.